A sealing agent or a gasket has hitherto been used for sealing an electronic part container for holding a precision electronic circuit element or a precision electronic part, such as, for example, a magnetic hard disc drive apparatus, in order to protect the precision electronic circuit element or the precision electronic part from interference caused by penetration of dusts and humidity.
In recent years, as the above-mentioned sealing agent and the gasket, in order for reducing the plant investment and the processing cost, gaskets are most widely used which are prepared by applying to a member an active ray-curable sealing agent composition using a dispenser or other means, and then irradiating the thus-applied sealing agent composition with ultraviolet ray. As the active ray-curable sealing agent composition, those which are predominantly comprised of an urethane acrylate oligomer having a low hardness and a high flexibility are used for obtaining a sealability sufficient for a gasket (see, for example, international publication WO96/10594).
An urethane acrylate oligomer has a structure such that a polyol ingredient such as polyester polyol, polyether polyol or polycarbonate polyol, a diisocyanate ingredient, and a hydroxyl group-containing monomer ingredient having radically polymerizable unsaturation are chemically bonded together.
In the case when using the above-mentioned urethane acrylate oligomer as the main ingredient of an active energy ray-curable sealing agent composition for sealing an electronic part container such as a magnetic hard disc drive apparatus, a resin structure comprised of repeating units in the urethane acrylate oligomer must be highly polymerized for obtaining a sealing agent having a low hardness, and enhanced flexibility and elongation. However, when the resin structure comprised of the urethane acrylate oligomer is highly polymerized, the content of a radically polymerizable unsaturated group-containing monomer ingredient having a hydroxyl group bonded to a terminal of the resin structure, in the urethane acrylate oligomer, is reduced, namely, the degree of unsaturation in the urethane acrylate oligomer is reduced. This reduction of the degree of unsaturation leads to deterioration in curing characteristics such as reduction in the fate of curing and undercure of the active ray-curable sealing agent composition predominantly comprised of the urethane acrylate oligomer, and further insufficient crosslinking density thereof. The deterioration in curing characteristics tends to cause decrease of performance quality such as physical and mechanical strength and durability of the active energy ray-cured sealing agent composition.